1. Field of the Invention
The present invention relates to a papermakers fabric for supporting and conveying fibrous webs through papermaking processes and more particularly to a papermakers fabric formed from a plurality of extruded slotted elements.
2. Description of the Prior Art
In papermaking machines, a papermakers fabric in the form of an endless belt-like structure is supported on and advanced by various machine rolls during the papermaking process. Papermaking fabrics carry various names depending on their intended use. By way of example, papermakers fabrics include forming fabrics, wet press felts, and dryer felts and fabrics, to name a few.
Forming fabrics, for example fourdrinier wires, or forming media, are commonly configured from a length of woven fabric with its ends joined together in a seam to provide an endless belt. The fabric may also be constructed by employing an endless weave process, thereby eliminating the seam. Either fabric generally comprises a plurality of machine direction yarns and a plurality of cross machine direction yarns which have been woven together on a suitable loom.
Recently, in the papermaking field, it has been found that synthetic materials may be used in whole or in part to produce forming fabrics of superior quality. Today, almost all forming fabrics are made from polyesters, such as Dacron or Trevira, acrylic fibers, such as Orlon, Dynel, and Acrylan, copolymers such as Saran, or polyamides, such as Nylon. The warp and weft yarns of the forming fabric may be of the same or different constituent materials and construction, and may be in the form of a monofilament or multifilament yarn.
Among the problems encountered in conventionally woven forming fabrics is edge curling and the trade-off between fabric strength and stability on the one hand and permeability on the other.
A conventional dryer felt consists of a woven endless conveyor belt made from a two- or three-plane fabric wherein the various planes are defined by different groups of cross machine direction yarns. The planes, plys or layers are united by a plurality of machine direction yarns. The yarns used to weave the most up-to-date dryer felts are made from synthetic monofilaments or synthetic multifilaments, from such materials as polyester or polyamide.
A further disadvantage of the woven papermakers belts is that there is no easy way to repair a damaged belt. Typically, if a papermakers belt is damaged in use, it must be replaced in its entirety.
Extruded synthetic elements have been used to form conveyor belts used in various material handling applications involving heavy, bulky articles. The extruding process provides a method of manufacture which is easy and inexpensive. Structural elements resulting from the extrusion process are then linked together one to the next, or by means of pintles, to form a strong, resilient belt.
Although the use of joined extruded elements is known in the field of conveyor belts, such belts have not been used in papermaking because they have not had the requisite characteristics for supporting and conveying wet, relatively fragile paper and fibrous webs. For example, many conveyor applications require that the article-contacting surface of the belt have ridges or other protrusions to provide gripping means to help hold the material on the belt. In contrast, in papermaking operations the surface of the papermakers belt must be smooth in order to minimize paper marking problems and to impart a smooth surface to the paper sheet. Current methods of joining extruded slotted elements typically leave a gap between one element and the next; such an arrangement would tend to produce paper with significant and undesirable markings caused by these joints. Additionally, such gaps permit the accumulation of dirt and foreign matter which could be transferred to the surface of the paper web.
A further disadvantage of the extruded conveyor belts used in material handling applications is the lack of controlled permeability of the blets. The papermaking process involves the removal of large amounts of water from the fiber-water slurry in the forming stage, and requires significant moisture transfer through the dryer fabrics used in the drying sections of such machines. Current extruded elements formed from synthetic materials are typically impermeable.
There is thus a need for a papermakers fabric which can function reliably in the various environments encountered during the papermaking process while at the same time being capable of being produced inexpensively and efficiently. It is also desirable that the papermakers fabric be endless in construction, have acceptable permeability, and be easily repaired while in position on a papermaking machine. The present invention is directed toward meeting those requirements.